1. Field of the Invention
The present invention relates to a piezoelectric-driven micro-electro-mechanical system (MEMS) device having piezoelectric drive mechanisms using a piezoelectric thin film and a method for manufacturing the same.
2. Related Art
Conventionally, a variable capacitor diode utilizing a change in thickness of the depletion layer of a PN junction have been used as a variable capacitor. The variable capacitor diode, however, has limited applications due to a capacitance change of only on the order of at most 5 times, and a small Q factor of on the order of 20-30 where a higher Q factor indicates less loss.
On the other hand, a variable capacitor fabricated using MEMS technology has recently attracted attention. A microswitch or a variable capacitor according to conventional MEMS technology utilizes an electrostatic force as a driving force. A MEMS device utilizing an electrostatic force has an advantage of a simple operating mechanism in which it can be operated only by applying a driving voltage to a pair of spaced electrodes (for example, see “Smart Structure and Materials 2002: Smart Electronics, MEMS and Nanotechnology”, V. K. Varadan, Editor, Proceedings of SPIE Vol. 4700 (2002), pp. 40-49).
On the contrary, the electrostatic force being essentially inversely proportional to the square of the distance results in a phenomenon called pull-in in which the space is discontinuously closed within the two-third of the initial space where the relationship between the applied voltage and the travel of electrons is nonlinear. The operating range is therefore narrow and generally a driving voltage of 20 volts or greater is required to move the electrodes a distance of 1 μm or greater. Under the present situation, the problem of high driving voltage hinders the applications in general consumer products.
Consequently, a variable capacitor can be conceived which has a movable electrode of the variable capacitor on a beam supported in the air above a substrate and provided with a piezoelectric thin film, and a fixed electrode of the variable capacitor on the confronting substrate, and in which the beam is driven with a piezoelectric force to change the distance between the movable and fixed electrodes.
Conventional methods for fabricating a beam structure supported in the air using MEMS technology include a method that involves fabricating a beam structure on a Si layer of an SOI substrate and removing part of the Si layer to thereby support the beam structure in the air, and a method that involves fabricating a beam structure on a Si substrate and forming a trench penetrating through the surface of the substrate by wet etching or RIE from the back side of the substrate to thereby support the beam structure in the air. Although these methods can create a movable beam and a movable electrode, it is difficult to form a fixed opposing electrode on the underlying substrate.
Alternatively, a fixed electrode is first formed on a substrate; then a sacrificial layer is formed thereon; a movable beam and a movable electrode are then formed so that they are bridged over the sacrificial layer; and then the sacrificial layer is removed, so that a beam structure supported in the air is fabricated. In this manner, a cantilever beam or fixed beam bridge structure having bend portions on the substrate is fabricated.
In the case of a cantilever beam structure, there is a problem that a slight residual stress in the material forming the beam causes the beam to warp upward or downward, and it is thereby significantly difficult to fabricate the structure with a constant distance between the movable and fixed electrodes, resulting in inconsistent manufacturing.
In the case of a fixed beam bridge structure, the fabrication can be performed somewhat more consistent than in a cantilever beam structure. Again however, there is a problem that a compression residual stress in the film material forming the beam causes the beam to warp upward or a tensile residual stress conversely causes the beam to warp downward, and it is thereby also significantly difficult to fabricate the structure with a constant distance between the movable and fixed electrodes, resulting in inconsistent manufacturing.
Also, both the cantilever and fixed beam structures have a common reliability problem, as described later, such that drive electrodes forming the piezoelectric drive mechanism are cut and shifted or the beam is cracked at the bent structures formed on the corners of the sacrificial layer.